Semiconductor device having an alignment layer with mask pits

ABSTRACT

A semiconductor device includes a gate structure extending from a first surface of a semiconductor portion into a mesa section between neighboring field electrode structures and an alignment layer formed on the first surface. The alignment layer includes mask pits formed in the alignment layer in a vertical projection of the field electrode structures. Sidewalls of the mask pits have a smaller tilt angle with respect to the first surface than sidewalls of the field electrode structures. The gate structure is in the vertical projection of a gap between neighboring mask pits.

BACKGROUND

The present application claims priority under 35 U.S.C. § 119 to German(DE) Patent Application Serial No. DE 10 2014 115 321.9 filed on Oct.21, 2014. The disclosure in this priority application is herebyincorporated fully by reference into the present application.

BACKGROUND ART

Power semiconductor devices such as power semiconductor diodes, IGFETs(insulated gate field effect transistors) and IGBTs (insulated gatebipolar transistors) are typically vertical devices with a load currentflow between a first surface at a front side of a semiconductor die anda second surface at the opposite rear side. Compensate on structuresextending from the front side into the semiconductor die deplete a driftzone formed in the semiconductor die in a blocking mode. Thecompensation structures allow higher dopant concentrations in the driftzone without adverse impact on the blocking capabilities.

It is desirable to improve the device characteristics of semiconductordevices.

SUMMARY

According to an embodiment a method of manufacturing a semiconductordevice includes forming first trenches extending from a process surfaceinto a semiconductor layer. An alignment layer with mask pits in a, withrespect to the process surface, vertical projection of the firsttrenches is formed on the process surface. Sidewalls of the mask pitshave a smaller tilt angle with respect to the process surface thansidewalls of the first trenches. The mask pits are filled with anauxiliary material. A gate trench for a gate structure is formed in amesa section between the first trenches, wherein the auxiliary materialis used as an etch mask.

According to another embodiment a semiconductor device includes a gatestructure extending from a first surface of a semiconductor portion intoa mesa section between neighboring field electrode structures. Analignment layer is formed on the first surface, wherein the alignmentlayer comprises mask pits in a with respect to the first surfacevertical projection of portions of the field electrode structures.Sidewalls of the mask pits have, a smaller tilt angle with respect tothe process surface than sidewalls of the field electrode structures.The gate structure is in the vertical projection of a gap betweenneighboring mask pits.

Those skilled in the art will recognize additional features andadvantages upon reading the following detailed description and onviewing the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide a furtherunderstanding of the invention and are incorporated in and constitute apart of this specification. The drawings illustrate the embodiments ofthe present invention and together with the description serve to explainprinciples of the invention. Other embodiments of the invention andintended advantages will be readily appreciated as they become betterunderstood by reference to the following detailed description.

FIG. 1A is a schematic cross-sectional view of a portion of asemiconductor substrate for illustrating a method of manufacturing asemiconductor device according to an embodiment, after forming firsttrenches.

FIG. 1B is a schematic cross-sectional view of the semiconductorsubstrate portion of FIG. 1A, after forming an alignment layer with maskpits in the vertical projection of the first trenches.

FIG. 1C is a schematic cross-sectional view of the semiconductorsubstrate portion of FIG. 1B after forming a gate trench self-aligned tothe first trenches.

FIG. 2 is a schematic cross-sectional view of a portion of asemiconductor device obtained by the method of manufacturing asillustrated in FIGS. 1A to 1C according to an embodiment.

FIG. 3A is a schematic cross-sectional view of a portion of asemiconductor substrate for illustrating a method of manufacturing asemiconductor device according to an embodiment concerning compensationsstructures, after forming field electrode structures.

FIG. 3B is a schematic cross-sectional view of the semiconductorsubstrate portion of FIG. 3A, after forming source and body wells.

FIG. 3C is a schematic cross-sectional view of the semiconductorsubstrate portion of FIG. 3B, after forming recesses in fielddielectrics in a transistor cell region.

FIG. 3D is a schematic cross-sectional view of the semiconductorsubstrate portion of FIG. 3C, after forming body contact zones.

FIG. 3E is a schematic cross-sectional view of the semiconductorsubstrate portion of FIG. 3D, after depositing conductive material inthe recesses to form buried contacts.

FIG. 3F is a schematic cross-sectional view of the semiconductorsubstrate portion of FIG. 3E, after forming an alignment layer with maskpits in the vertical projection of the buried contacts.

FIG. 3G is a schematic cross-sectional view of the semiconductorsubstrate portion of FIG. 3F, after filling the mask pits with anauxiliary material.

FIG. 3H is a schematic cross-sectional view of the semiconductorsubstrate portion of FIG. 3G, after forming mask openings in thealignment layer between the mask pits.

FIG. 3I is a schematic cross-sectional view of the semiconductorsubstrate portion of FIG. 3H, after forming gate trenches in thevertical projection of the mask openings.

FIG. 3J is a schematic cross-sectional view of the semiconductorsubstrate portion of FIG. 3I, after forming gate structures in the gatetrenches.

FIG. 3K is a schematic cross-sectional view of the semiconductorsubstrate portion of FIG. 3J, after forming an etch mask for fieldelectrode contacts.

FIG. 4A is a schematic cross-sectional view of a portion of asemiconductor device including field electrode structures obtained bythe method illustrated in FIGS. 3A to 3K.

FIG. 4B is a schematic horizontal cross-sectional view of thesemiconductor device of FIG. 4A along line I-I according to anembodiment related to stripe-shaped gate and field electrodes.

FIG. 4C is a schematic horizontal cross-sectional view of thesemiconductor device of FIG. 4A along line I-I according to anembodiment related to stripe-shaped gate electrodes and spicular fieldelectrodes arranged in lines.

FIG. 4D is a schematic horizontal cross-sectional view of thesemiconductor device of FIG. 4A along line I-I according to anembodiment related to a grid-shaped gate structure with spicular fieldelectrode structures formed in the meshes of the grid.

FIG. 4E is a schematic vertical cross-sectional view of a portion of atransition region of the semiconductor device of FIG. 4A in a planeparallel to the cross-sectional plane of FIG. 4A according to anembodiment related to self-aligned gate contacts.

FIG. 5 is a simplified flowchart of a method for manufacturing asemiconductor device according to a further embodiment.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof and in which are shownby way of illustrations specific embodiments in which the invention maybe practiced. It is to be understood that other embodiments may beutilized and structural or logical changes may be made without departingfrom the scope of the present invention. For example, featuresillustrated or described for one embodiment can be used on or inconjunction with other embodiments to yield yet a further embodiment. Itis intended that the present invention induces such modifications andvariations. The examples are described using specific language, whichshould not be construed as limiting the scope of the appending claims.The drawings are not scaled and are for illustrative purposes only. Forclarity, the same elements have been designated by correspondingreferences in the different drawings if not stated otherwise.

The terms “having”, “containing”, “including”, “comprising” and the likeare open, and the terms indicate the presence of stated structures,elements or features but do not preclude additional elements orfeatures. The articles “a”, “an” and “the” are intended to include theplural as well as the singular, unless the context clearly indicatesotherwise.

The term “electrically connected” describes a permanent low-ohmicconnection between electrically connected elements, for example a directcontact between the concerned elements or a low-ohmic connection via ametal and/or highly doped semiconductor. The term “electrically coupled”includes that one or more intervening element(s) adapted for signaltransmission may be provided between the electrically coupled elements,for example elements that are controllable to temporarily provide alow-ohmic connection in a first state and a high-ohmic electricdecoupling in a second state.

The Figures illustrate relative doping concentrations by indicating “−”or “+” next to the doping type “n” or “p”. For example, “n⁻” indicates adoping concentration which is lower than the doping concentration of an“n”-doping region, whereas an “n⁺”-doping region has a higher dopingconcentration than an “n”-doping region. Doping regions of the samerelative doping concentration do not necessarily have the same absolutedoping concentration. For example, two different “n”-doping regions mayhave the same or different absolute doping concentrations.

FIGS. 1A to 1C illustrate a sequence of processes providing gatestructures self-aligned to previously formed subsurface structures,wherein the subsurface structures may be compensation structures orportions of compensation structure, by way of example. The term“self-aligned” as used in the following indicates that the position ofthe gate structures in relation to the subsurface structures is notsubject to a possible miss-alignment between two or morephotolithographic masks. Instead, the position of the gate structuresrelative to the subsurface structures is defined bynon-photolithographic patterning processes, which include deposition andetch processes.

The semiconductor substrate 500 a illustrated in FIG. 1A consists of orincludes a semiconductor layer 100 a of a single-crystallinesemiconductor material. The single-crystalline semiconductor materialmay be silicon (Si), silicon carbide (SiC), germanium (Ge), asilicon-germanium crystal (SiGe), gallium nitride (GaN) or galliumarsenide (GaAs), or another A_(III)B_(V) semiconductor, by way ofexample. The semiconductor substrate 500 a may include furthersemiconductor and dielectric layers in addition to the semiconductorlayer 100 a. According to an embodiment, the semiconductor substrate 500a is a silicon wafer, wherein the semiconductor layer 100 a may be asilicon disk obtained by cutting a silicon crystal or entirely or inportions grown by epitaxy on a single-crystalline substrate. Thesemiconductor layer 100 a may include two or more sub-layers ofdifferent conductivity types or of the same conductivity type butdiffering in a mean dopant concentration.

The semiconductor layer 100 a forms a planar process surface 101 a at afront side of the semiconductor substrate 500 a. A normal to the processsurface 101 a defines a vertical direction. Directions orthogonal to thevertical direction are horizontal directions.

First trenches 162 extend from a plane spanned by the process surface101 a into the semiconductor layer 100 a. Portions of the semiconductorlayer 100 a between neighboring first trenches 162 form mesa sections170. A width wR of the first trenches 162 and a distance wM betweenneighboring first trenches 162 are defined by the appliedphotolithography process.

The first trenches 162 may be trenches etched into the semiconductorlayer 100 a. According to other embodiments the first trenches 162 maybe formed by recessing subsurface structures 160 a or portions ofsubsurface structures 160 a previously formed in the semiconductor layer100 a. The subsurface structures 160 a may be conductive structures, forexample contact structures, or insulator structures, for example deviceisolations. According to other embodiments, the subsurface structures160 a are control structures made up of sub-structures of differentmaterials such as a first set of gate structures, auxiliary controlstructures or field electrode structures. The first trenches 162 may beformed by recessing the complete subsurface structures 160 a or byrecessing portions of the subsurface structures 160 a. According to anembodiment, the subsurface structures 160 a are field electrodestructures including a conductive field electrode as well as a fielddielectric separating the field electrode from the semiconductormaterial of the semiconductor layer 100 a, wherein the first trenches162 may be formed at least in portions of the field dielectrics.

According to the illustrated embodiment, the first trenches 162 areformed by etching recesses 162 a with a vertical extension vPR into thesubsurface structures 160 a, filling lower portions of the recesses 162a with a conductive material, and recessing the deposited conductivematerial to form buried contacts 163 in a distance to the processsurface 101 a as well as the first trenches 162 with a verticalextension vR between the buried contacts 163 and the process surface 101a.

FIG. 1A shows the subsurface structures 160 a on opposite sides of anintermediate mesa section 170. First trenches 162 extend from theprocess surface 101 a down to buried contacts 163 at a distance to theprocess surface 101 a given by the vertical extension vR of the firsttrenches 162. The buried contacts 163 extend between a first distancegiven by the vertical extension vR of the first trenches 162 and asecond distance to the process surface 101 a given by the verticalextension vPR of the recesses 162 a. The vertical extension vR of thefirst trenches 162 is in a range from 50 nm to 300 nm, for example in arange from 100 nm to 200 nm. According to an embodiment, the verticalextension vR may be between 150 nm and 170 nm. The horizontal width wRof the first trenches 162 may be in a range from 50 nm to 200 nm, forexample in a range from 100 nm to 150 nm.

An alignment layer 210 that covers the process surface 101 a and thatfills the first trenches 162 is formed in a way that mask pits 211 arein situ formed in the alignment layer 210 in the vertical projection ofthe first trenches 162. According to an embodiment, the alignment layer210 is formed by a deposition process with a conformal depositioncomponent and a sputter component smoothing steep edges in the depositedlayer. According to an embodiment, a process for forming the alignmentlayer 210 combines highly conformal deposition from the gaseous phasewith sputtering based on, by way of example, low-energy ions that removedeposited material along exposed steep edges more rapidly than fromother regions. According to an embodiment, an HDP (high density plasma)process deposits silicon oxide to form the alignment layer 210. Duringdeposition, a sputter process may be based on arsenic ions, oxygen ionsor SiH₄ ions.

As shown in FIG. 1B, the alignment layer 210 covers the process surface101 a and completely fills the first trenches 162. The mask pits 211 areformed in the vertical projection of the buried contacts 163. A verticalextension vM of the mask pitches 211 is approximately equal to thevertical extension vR of the first trenches 162. A tilt angle α betweensidewalls 212 of the mask pits 211 and the process surface 101 a issmaller than a tilt angle β between the sidewalls of the first trenches162 and the process surface 101 a. The tilt angle α may be in a rangefrom 30 degree to 60 degree, for example in a range from 40 to 45degree. According to an embodiment, the tilt angle α is approximately 43degree.

A thickness vAL of the alignment layer 210 is set considering the mesawidth wM between neighboring subsurface structures 160 a, the tilt angleα and a target distance between the subsurface structures 160 a and thegate structures 150 and may be greater than the vertical extension vM ofthe mask pits 211. For example, the thickness vAL of the alignment layer210 is selected such that an exposed surface of the alignment layer 210has a horizontal portion 218 in the vertical projection of the mesasection 170, wherein the horizontal portion 218 is parallel to theprocess surface 101 a and has a horizontal width wH of at least 20 nm,for example at least 40 nm.

An auxiliary material different from the material of the alignment layer210 is deposited to completely fill the mask pits 211. Portions of theauxiliary material outside the mask pits 211 are removed, for exampleusing a CMP (chemical mechanical polishing) process slopping at thehorizontal portions 218 of the exposed surface of the alignment layer210. According to an embodiment, after the CMP the deposited andpolished auxiliary material may be slightly recessed in order to adjustthe distance between neighboring portions of the deposited auxiliarymaterial and to compensate for process fluctuations concerning thealignment layer 210. The auxiliary material may be any material againstwhich the material of the alignment layer 210 and, optionally, thesemiconductor layer 100 a may be etched with high selectivity. Forexample, the auxiliary material may be amorphous silicon, carbon, or aphotoresist. According to an embodiment, the auxiliary material ispolycrystalline silicon.

The polished and, optionally, recessed auxiliary material forms an etchmask 430 or a portion of an etch mask for forming a gate trench 152 inthe semiconductor mesa 170. For example, in a first step, a mask opening214 may be formed by an etch process removing the material of thealignment layer 210 selectively with respect to the auxiliary materialof the etch mask 430. In a second step, the gate trench 152 is formed inthe semiconductor layer 100 a in a portion exposed by the mask opening214.

FIG. 1C shows the gate trench 152 formed in a self-aligned manner in thecenter of the semiconductor mesa 170 between neighboring subsurfacestructures 160 a. When, according to a comparative example, the gatetrenches 152 are formed by a second photolithography mask, alignmenttolerances of some 10 nanometers have to be considered when selectingthe mesa width wM in order to ensure a minimum distance between the gatetrenches 152 and the subsurface structures 160 a. Using the self-alignedapproach as illustrated, no alignment tolerance has to be consideredsuch that a smaller mesa width wM can be obtained at high yield. Inaddition, the self-aligned approach may save an expensivephotolithography process.

According to embodiments including the formation of the buried contacts163, conductive structures in the mesa section 170 can be electricallyconnected to conductive structures in the subsurface structure 160 awithout any contact structures consuming material of the mesa section170.

The following Figures refer to semiconductor devices with n-channelIGFET cells. Equivalent considerations apply to p-channel IGFET cellswith a complementary doping.

FIG. 2 illustrates a semiconductor device 500 including a plurality ofidentical transistor cells TC. The semiconductor device 500 may be ormay include an IGFET, for example an MOSFET (metal oxide semiconductorFET) in the usual meaning including FETs with metal gates as well asFETs with non-metal gates. According to another embodiment, thesemiconductor device 500 may be an IGBT or an MGD (MOS gated diode).

The semiconductor device 500 is based on a semiconductor portion 100from a single crystalline semiconductor material such as Si, SiC, Ge,SiGe, GaN, GaAs or any other A_(III)B_(V) semiconductor and with a firstsurface 101 at a front side and a second surface 102 at an opposite rearside.

The semiconductor portion 100 includes a drift and rear side structure120 with a heavily doped contact portion 129 formed along the secondsurface 102. The drift and rear side structure 120 includes a drift zone121, in which a dopant concentration may gradually or in steps increaseor decrease with increasing distance to the first surface 101 at leastin portions of its vertical extension. According to other embodiments,the dopant concentration in the drift zone 121 may be approximatelyuniform. A mean dopant concentration in the drift zone 121 may bebetween 1E13 cm⁻³ and 1E17 cm⁻³, for example in a range from 5E15 cm⁻³to 5E16 cm⁻³. The drift and rear side structure 120 may include furtherdoped zones, for example a field stop layer 128 that separates the driftzone 121 from the contact portion 129. A mean dopant concentration inthe field stop layer 128 may be at least five times as high as a meandopant concentration in the drift zone 121 and at most one-fifth of amaximum dopant concentration in the contact portion 129.

The contact portion 129 may be a heavily doped base substrate or aheavily doped layer. Along the second surface 102 a dopant concentrationin the contact portion 129 is sufficiently high to form an ohmic contactwith a metal directly adjoining the second surface 102. In case thesemiconductor portion 100 is based on silicon, in an n-conductivecontact portion 129 the dopant concentration along the second surface102 may be at least 1E18 cm⁻³, for example at least 5E19 cm⁻³. In ap-conductive contact portion 129, the dopant concentration may be atleast 1E16 cm⁻³, for example at least 5E17 cm⁻³.

Each transistor cell TC includes a field electrode structure 160 or aportion of a field electrode structure 160 extending from the firstsurface 101 into the semiconductor portion 100. Portions of the fieldelectrode structure 160 between the first surface 101 and a buried endportion may have approximately vertical sidewalls or may slightly taperwith respect to the first surface 101. The sidewalls may be straight orslightly bulgy.

The field electrode structures 160 may include a stripe-shaped fieldelectrode 165 or a spicular or needle-shaped field electrode 165 as wellas a field dielectric 161 surrounding the field electrode 165,respectively. The field electrode 165 includes or consists of a heavilydoped polycrystalline silicon layer and/or a metal-containing layer. Thefield dielectric 161 separates the field electrode 165 from thesurrounding semiconductor material of the semiconductor portion 100 andmay include or consist of a thermally grown silicon oxide layer, adeposited silicon oxide layer, e.g. a silicon oxide based on TEOS(tetraethyl orthosilicate), or both.

A vertical extension of the field electrode structures 160 may be in arange from 1 μm to 50 μm, for example in a range from 2 μm to 20 μm. Afirst horizontal extension of the field electrode 165 may be at mostthree times or at most twice as large as a second horizontal extensionorthogonal to the first horizontal extension. The second horizontalextension may be in a range from 0.1 μm to 20 μm, for example in a rangefrom 0.2 μm to 5 μm.

The cross-sectional areas of the field electrodes 165 and the fieldelectrode structures 160 may be rectangles, or regular or distortedpolygons, with or without rounded and/or beveled corners, respectively.According to an embodiment, the first and second horizontal extensionsare approximately equal and the cross-sectional areas of the fieldelectrodes 165 rand the field electrode structures 160 are regularpolygons such as octagons, hexagons or squares, with or without roundedor beveled corners, respectively.

According to other embodiments, the cross-sectional areas of the fieldelectrodes 165 and the field electrode structures 160 may be ellipses orovals. The first and second horizontal extensions may be approximatelyequal such that the horizontal cross-sectional areas of the fieldelectrodes 165 and the field electrode structures 160 are circles. Incase of specular field electrodes 165, the transistor cells TC may bearranged matrix-like in lines and rows. According to other embodiments,the transistor cells TC may be arranged in shifted lines, wherein theodd lines are shifted with respect to the even lines by half thedistance between two transistor cells TC along the line.

Semiconducting portions of the transistor cells TC are formed in themesa sections 170 of the semiconductor portion 100, wherein the mesasections 170 may surround the respective field electrode structure 160.The mesa sections 170 protrude from a contiguous section of thesemiconductor portion 100. A horizontal minimum width wM of the mesasection 170 may be in a range from 0.3 μm to 2 μm, for example in arange from 0.4 μm to 1 μm. Each mesa section 170 may include a body zone115 forming a first pn junction pn1 with a section of the drift zone 121in the concerned mesa section 170 as well as one or more source zones110 forming second pn junctions pn2 with the body zones 115. The bodyzones 115 separate the source zones 110 from the drift zone 121.

A gate structure 150 includes a conductive gate electrode 155 that mayinclude or consist of a heavily doped polycrystalline silicon layerand/or a metal-containing layer. The gate electrode 155 is completelyinsulated against the semiconductor portion 100, wherein a gatedielectric 151 separates the gate electrode 155 at least from the bodyzone 115. The gate dielectric 151 capacitively couples the gateelectrode 155 to channel portions of the body zones 115. The gatedielectric 151 may include or consist of a semiconductor oxide, forexample thermally grown or deposited silicon oxide, semiconductornitride, for example deposited or thermally grown silicon nitride, asemiconductor oxynitride, for example silicon oxynitride, or acombination thereof. The gate structure 150 may be a trench gateextending from the first surface 101 into the semiconductor portion 100.

In the illustrated embodiments and for the following description, thetransistor cells TC are n channel IGFET cells with p doped body zones115 and n doped source and drift zones 110, 121. Similar considerationsas outlined below apply also to embodiments including p channel IGFETcells with a complementary doping.

When a voltage applied to the gate electrode 150 exceeds a presetthreshold voltage, electrons accumulate in the channel portions of thebody zones 115 directly adjoining the gate dielectric 151 and forminversion channels making the second pn junctions pn2 permeable forelectrons.

A vertical extension of the gate structures 150 is smaller than thevertical extension of the field electrode structures 160. The verticalextension of the gate structures 150 may be in a range from 100 nm to5000 nm, for example in a range from 300 nm to 1000 nm.

An interlayer dielectric 200 includes an alignment layer 210 formed onthe first surface 101 and the field electrode structures 160. Thematerial of the alignment layer 210 may be a semiconductor oxide, forexample silicon oxide deposited in a HDP process. The alignment layer210 includes mask pits 211 in the vertical projection of at leastportions of the field electrode structures 160. According to anembodiment, the mask pits 211 are formed in the vertical projection ofthe field dielectrics 161. Sidewalls 212 of the mask pits 211 have asmaller tilt angle α with respect to the first surface 101 thansidewalls of the field electrode structures 160. Between neighboringmask pits 211 a mask opening 214 in the alignment layer 210 extends in avertical direction through the alignment layer 210. The mask opening 214is in the vertical projection of the gate structure 150.

The interlayer dielectric 200 further includes a dielectric layer 220formed on the alignment layer 210 and filling the mask pits 211 as wellas the mask openings 214. The dielectric layer 220 may include one ormore insulator layers from silicon oxide, silicon nitride, siliconoxynitride, doped or undoped silicate glass, for example BSG (boronsilicate glass), PSG (phosphorus silicate glass) or BPSG (boronphosphorus silicate glass), by way of example.

A first load electrode 310 formed on the interlayer dielectric 200 mayform or may be electrically coupled or connected to a first loadterminal, for example a source terminal in case the semiconductor device500 is an IGFET, an emitter terminal in case the semiconductor device500 is an IGBT or an anode terminal in case the semiconductor device 500is an MGD. A second load electrode 320, which directly adjoins thesecond surface 102 and the contact portion 129, may form or may beelectrically connected to a second load terminal, which may be the drainterminal in case the semiconductor device 500 is an IGFET, the collectorterminal in case the semiconductor device 500 is an IGBT or a cathodeterminal in case the semiconductor device 500 is a MGD.

Each of the first and second load electrodes 310, 320 may consist of orcontain, as main constituent(s), aluminum (Al), copper (Cu), or alloysof aluminum or copper, for example AlSi, AlCu or AlSiCu. According toother embodiments, at least one of the first and second load electrodes310, 320 may contain, as main constituent(s), nickel (Ni), tin (Sn),titanium (Ti), tungsten (W), tantalum (Ta), vanadium (V), silver (Ag),gold (Au), platinum (Pt), and/or palladium (Pd). For example, at leastone of the first and second load electrodes 310, 320 may include two ormore sub-layers, wherein each sub-layer contains one or more of Ni, Sn,Ti, V, Ag, Au, Pt, W, and Pd as main constituent(s), e.g., a silicide, anitride and/or an alloy.

At a distance to the first surface 101 a buried contact 163 in thevertical projection of at least a portion of a field dielectric 161directly adjoins the field electrode 165 at one side and the body zone115 or the body zone 115 as well as the source zone 110 at the oppositeside.

Contact structures 315 extend through openings in the interlayerdielectric 200 and electrically connect the first load electrode 310with the field electrodes 165 and, through the buried contacts 163 withthe source and body zones 110, 115 of the transistor cells TC. Theburied contacts 163 as well as the contact structures 315 may includeone or more conductive metal containing layers based on, e.g., titanium(Ti) or tantalum (Ta) and a metal fill portion, e.g., based on tungsten(W). According to other embodiments the contact structures 315 mayinclude heavily doped polycrystalline structures. The gate structure 150is formed in the center of the mesa cell 170. With the mask pits 211self-aligned to the field electrode structures 160 and the gatestructure 150 self-aligned to the mask pits 211, no mask alignmenttolerances have to be considered when aligning the gate structures 150to the field electrode structures 160. As a consequence, the mesa widthwM can be significantly reduced compared to approaches using differentphotolithographic masks for defining the field electrode structures 160on the one hand and the gate structures 150 on the other hand.

In addition, the buried contact structures 163 may spare a surfacecontact of the source zones 110 and the body zones 115 that may consumemesa area. As a result, the mesa width wM between neighboring fieldelectrode structures can be reduced to below 800 nm, for example tobelow 500 nm at high yield. A narrow mesa width wM may improve atrade-off between breakdown performance and on-state resistance, e.g.,for semiconductor devices specified for a comparatively low breakdownvoltage.

FIGS. 3A to 3K refer to a method of manufacturing semiconductor deviceswith gate structures 150 formed self-aligned to field electrodestructures 160 used as the subsurface structures 160 a of FIGS. 1A to1C.

A semiconductor substrate 500 a consists of or includes a semiconductorlayer 100 a of a single-crystalline semiconductor material. Thesemiconductor substrate 500 a may be a semiconductor wafer from which aplurality of identical semiconductor dies is obtained. Thesingle-crystalline semiconductor material of the semiconductor layer 100a may be Si, SiC, Ge, SiGe, GaN, GaAs or any other A_(III)B_(V)semiconductor. The semiconductor layer 100 a may be intrinsic or lightlydoped. According to an embodiment, the semiconductor layer 100 a islightly n-doped. For example, the semiconductor layer 100 a containsphosphorus (P) and/or arsenic (As) atoms. A planar process surface 101 aof the semiconductor layer 100 a is exposed at a front side. At anopposite rear side, the semiconductor substrate 500 a may have a planarrear side surface 102 a.

Field electrode structures 160 extend from the first surface 101 a intothe semiconductor layer 100 a down to a bottom plane BPL. Each fieldelectrode structure 160 includes a conductive stripe-shaped or spicularfield electrode 165 as well as a field dielectric 161 surrounding thefield electrode 165. The field electrodes 165 include or consist of aheavily doped polycrystalline silicon layer and/or a metal containinglayer. The field dielectrics 161 separate the field electrodes 165 fromthe surrounding semiconductor material of the semiconductor layer 100 aand may include or consist of a thermally grown silicon oxide layer, adeposited silicon oxide layer, e.g., a silicon oxide based on TEOS orboth. The cross-sectional areas of the field electrodes 165 and thefield electrode structures 160 may be stripes with a first horizontalextension exceeding at least ten times a second horizontal extensionperpendicular to the first horizontal extension. According to otherembodiments, the cross-sectional areas of the field electrodes 165 andthe field electrode structures 160 are dot-shaped with a firsthorizontal extension exceeding the second horizontal extension at mostthree times. For example, the cross-sectional areas may be rectangles,regular or distorted polygons such as hexagons or octagons with orwithout rounded and/or beveled corners, respectively. According to anembodiment, the first and second horizontal extensions are approximatelyequal and the cross-sectional areas of the field electrodes 165 and thefield electrode structures 160 are regular polygons such as octagons,hexagons or squares with or without rounded and/or beveled corners,respectively.

According to other embodiments, the cross-sectional areas of the fieldelectrodes 165 and the field electrode structures 160 may be ellipses orovals, or, in case the first and second horizontal extensions are equal,circles.

A minimum horizontal extension wFE of the field electrodes 165 may be ina range from 0.1 μm to 20 μm, for example in a range from 0.2 μm to 5μm. A distance between the process surface 101 a and the bottom planeBPL gives a vertical extension vFES of the field electrode structure 160and may be in a range from 1 μm to 50 μm, for example in a range from 2μm to 20 μm.

Buried bottom sections of the field electrode structures 160 may beapproximately horizontal or may be rounded. Vertical sidewall sectionsof the field electrode structures 160 between the process surface 101 aand the bottom section may be strictly vertical, may slightly taperand/or may be bulgy.

Portions of the semiconductor layer 100 a between neighboring fieldelectrode structures 160 form mesa sections 170. The mesa sections 170may be stripe-shaped or may form a grid embedding spicular fieldelectrode structures 160. A minimum mesa width wM may be in a range from300 nm to 1 μm, for example in a range from 400 nm to 800 nm.

A first mask layer is deposited and patterned by photolithography toform a first mask 410 exposing at least transistor cell regions 610surrounded by edge regions 690, respectively. Using the first mask 410as an implant mask dopants may be introduced for forming body wells 115a in the mesa sections 170 of the transistor cell regions 610. Accordingto an embodiment, acceptor atoms may be implanted at a distance to theprocess surface 101 a. Then the semiconductor substrate 500 a may betempered to anneal the implant damages and to diffuse the implanteddopants. Using the first mask 410 or another mask replacing the firstmask 410, donor atoms may be implanted close to the process surface 101a. The semiconductor substrate 500 a may be tempered to anneal theimplant damage and to diffuse out the implanted dopants to form sourcewells 110 a in the mesa sections 170 of the transistor cell regions 610.

FIG. 3B shows the first mask 410 covering the edge portion 690. In themesa sections 170 of the transistor cell regions 610 source wells 110 aare formed alone the process surface 101 a. Body wells 115 a separatethe source wells 110 a from the remaining portion of the semiconductorlayer 100 a forming a drift region 120 a. First pn junctions pn1 formedbetween the body wells 115 a and the drift region 120 a have a distancedPN1 to the process surface 101 a. The distance dPN1 may be in a rangefrom 800 nm to 2000 nm. Second pn junctions pn2 between the source wells110 a and the body wells 115 a may have a distance dPN2 to the processsurface 101 a. The distance dPN2 may be in a range from 100 nm to 800nm, for example from 200 nm to 600 nm.

The implants for the formation of the body and source wells 115 a, 110 amay use the same etch mask 410 as illustrated. According to anotherembodiment, the implant mask for the body wells 115 a may expose, inaddition to the transistor cell region 610, transition regions 691 ofthe edge regions 690, wherein the transition regions 690 directly adjointhe transistor cell regions 610 and wherein p wells 115 b are formed inthe transition area 691 along the process surface 101 a.

Using the first mask 410 of FIG. 3B or a second mask 420 replacing thefirst mask 410 as an etch mask, the field dielectrics 691 are recessedin the transistor cell region 610. For example, the recess is an oxideetch, e.g. a wet etch process selectively recessing the exposed portionsof the field dielectrics 161, wherein the recess starts from the planeof the process surface 101 a.

FIG. 3C shows recesses 162 a resulting from the recess of the fielddielectrics 161. A vertical extension vPR of the recesses 162 is greaterthan the distance dPN2 of the second pn junctions pn2 to the processsurface 101 a and smaller than the distance dPN1 of the first pnjunctions pn1 to the process surface 101 a. The recesses 162 a exposesidewalls of the mesa sections 170 as well as portions of the body wells115 a formed within the mesa sections 170.

Using the second mask 420 of FIG. 3C, the first mask 410 of FIG. 3B or afurther mask replacing the first or second mask 410, 420 as an implantmask, acceptor atoms may be implanted by an angled implant into exposedsidewall portions of the mesa sections 170.

FIG. 3D shows heavily doped body contact zones 115 c formed alongexposed sidewall portions of the body wells 115 a as well as a counterimplant zone 110 x along exposed surfaces of the source wells 110 a.

The mask used as the implant mask for forming the body contact zones 115c, e.g., the second mask 420 is removed. Conductive material may bedeposited to form buried contacts 163 in first portions of the recesses162 a. According to an embodiment, an interface layer 163 a containingTi and/or Ta, for example TaN, may be deposited that lines the recesses162 a. A conductive fill material 163 b, for example a metal such asTungsten is deposited to fill the remaining voids. The depositedmaterials may be commonly recessed.

FIG. 3E shows the buried contacts 163 formed from the deposited andrecessed conductive interface layer 163 a and the conductive fillmaterial 163 b. The buried contacts 163 directly adjoin the heavilydoped body contact zones 115 c as well as the field electrodes 165. Inthe vertical projection of the buried contacts 163 first trenches 162extend from a plane spanned by the process surface 101 a down to anexposed surface of the buried contact structures 163. A verticalextension vR of the first trenches 162 is smaller than the distance dPN2between the second pn junctions pn2 and the process surface 101 a.According to an embodiment, the vertical extension of the first trenches162 may be in a range from 100 nm to 160 nm.

Donor atoms may be implanted through exposed surfaces of the mesasections 170 to achieve a sufficient high donor concentration along theprocess surface 101 a to form ohmic contacts with a metal. For example,arsenic atoms may be implanted using an implant mask exposing thetransistor cell regions 610. According to another embodiment, heavilydoped polycrystalline silicon containing donors may be deposited atleast in the transistor cell regions 610 and the donors may be diffusedout from the polycrystalline layer into the mesa sections 170. Accordingto a further embodiment, the implant for forming the source wells 110 aprovides an implant dose which is sufficiently high to achieve thedesired quality of the ohmic contact to the metal even after the counterimplant for forming the body contact zones 115 c.

An alignment layer 210 is deposited on the process surface 101 a and inthe first trenches 162 in a way that in the vertical projection of thefirst trenches 162 mask pits 211 are in-situ formed in the alignmentlayer 210. According to an embodiment, a HDP process deposits siliconoxide to form the alignment layer 210. During deposition, the sputterpower may be in a range from 800 to 1200 Watt and the sputter atoms maybe Argon (Ar) atoms, oxygen atoms or SiH₄ molecules.

FIG. 3F shows details of the alignment layer 210 and the mask pits 211.The thickness vAL of the alignment layer 210 is selected such that afterdeposition, the alignment layer 210 completely fills the first trenches162, and such that a horizontal portion 218 of the alignment layer 210between neighboring field electrode structures 160 has a minimum widthof at least 20 nm, for example at least 40 nm. In the rest, thethickness vAL of the alignment layer 210 determines the distance betweenthe field electrode structures 160 and gate structures formed in themesa sections 170 in the vertical projection of the horizontal portion218 in the following. The thickness vAL of the alignment layer 210 maybe in a range from 200 nm to 250 nm, by way of example.

A tilt angle α between sidewalls 212 of the mask pits 211 and theprocess surface 101 a is in a range from 30 degree to 60 degree, forexample in a range from 40 to 45 degree. According to an embodiment, thetilt angle α is approximately 43 degree.

An auxiliary material different from the material of the alignment layer210 is deposited to completely fill the mask pits 211. The auxiliarymaterial deposited outside the mask pits 211 may be removed by apolishing step, for example by CMP. A further slight recess of theauxiliary material may compensate for deviations caused by processfluctuations of the deposition process for the alignment layer 210.

FIG. 3G shows the auxiliary material forming a first mask portion 431 ofa third mask. In the vertical projection of the mesa sections 170 awidth wH between neighboring sections of the first mask portion 431defines position and width of gate structures to be formed in thefollowing. The auxiliary material may be any material against which thematerial of the alignment layer 210 is selectively etchable. Accordingto an embodiment, the alignment layer 210 is a silicon oxide layer andthe auxiliary material is selected from polycrystalline silicon, carbon,photoresist materials and silicon nitride. A third mask layer isdeposited and patterned by photolithography to form a second maskportion 432 of the third mask 430.

As illustrated in FIG. 3H the second mask portion 432 may cover the edgeregions 690 and may expose the transistor cell regions 610. According tothe illustrated embodiment, openings in the second mask portion 432expose portions of the alignment layer 210 around the gaps in the firstmask portion 431. The material of the second mask portion 432 is anymaterial against which the material of the alignment layer 210 can beetched with high selectivity and is another material as the auxiliarymaterial forming the first mask portions 431. According to anembodiment, the second mask portion 432 includes or consists of siliconnitride, photoresist, or carbon.

Alignment tolerances for the openings in the second mask portion 432 arerelaxed since the first mask portions 431 ensure that the resulting maskopenings 214 in the alignment layer 210 are fine-adjusted in the centerof the mesa sections 170. Using the third mask 430 as a combined etchmask, mask openings 214 are etched into the alignment layer 210 in thevertical projection of the mesa sections 170.

The second mask portion 432 may be removed and gate trenches 152 may beetched into the mesa sections 170 exposed by the mask openings 214 inthe alignment layer 210. The auxiliary material forming the first maskportion 431 may be previously removed or may be consumed during theformation of the gate trenches 152. A gate dielectric 151 may be formedat least along exposed sidewalls of the mesa sections 170.

FIG. 3I shows the gate trenches 152 centered with respect to theneighboring field electrode structures 160. A vertical extension of thegate trenches 152 is greater than the distance dPN1 between the first pnjunction pn1 and the process surface 101 a. The gate trenches 152 extendthrough the source wells 110 a and body wells 115 a of FIG. 3H. Onopposing sides of the gate trenches 152, source zones 110 are formedfrom the source wells 110 a of FIG. 3H and body zones 115 are formedfrom the body wells 115 a of FIG. 3H. Gate dielectrics 151, for examplefrom thermally grown silicon oxide, line the gate trenches 152.

Conductive material may be deposited and recessed to form gateelectrodes 155 in the gate trenches 152. The conductive material may bea metal, a conductive metal compound, heavily doped polycrystallinesilicon or a combination thereof. The recess may include a CMP and a wetetch. Along sidewalls of the mask openings 214 in the alignment layer210 dielectric spacers may be formed from a material different from thematerial of the alignment layer 210. Then a further dielectric layer 220may be deposited for completing an interlayer dielectric 200.

FIG. 3J shows auxiliary spacers 230 lining the mask openings 214 in thealignment layer 210. The auxiliary spacers 230 may be formed fromsilicon nitride and may support the formation of self-aligned gatecontacts as described below with regard to FIG. 4E.

The dielectric layer 220 covers the alignment layer 210 and fills themask pits 211 as well as the mask openings 214 in the alignment layer210. The dielectric layer 220 may be or may include deposited siliconoxide, for example a silicon oxide based on TEOS, BSG, PSG, BPSG, or acombination thereof. In combination with the alignment layer 210 thedielectric layer 220 forms an interlayer dielectric 200. A fourth masklayer may be deposited and patterned by photolithography to form afourth mask 440.

FIG. 3K shows the fourth mask 440 with openings 442 in the verticalprojection of the field electrodes 165. Using the fourth mask 440 as anetch mask, contact trenches may be etched through the interlayerdielectric 200 to the field electrodes 165. The contact trenches may befilled with a conductive material to form contact structures. Conductivematerial, form example aluminum, copper or a combination of aluminum andcopper with or without silicon may be deposited to form a first loadelectrode at the front side defined by the process surface 101 a. Adrain structure and a second load electrode may be completed at the rearside to form devices as illustrated in the following figures.

FIGS. 4A to 4D refer to IGFETs 501 with heavily n-doped contact portions129 along the second surface 102 a. The transistor cells TC are formedin a transistor cell region 610. An edge area 690 without transistorcells TC surrounds the transistor cell area 610. For further details,reference is made to the description of FIG. 2 .

FIG. 4B refers to an embodiment with stripe-shaped gate structures 150and stripe-shaped field electrode structures 160.

According to the embodiment of FIG. 4C, needle-shaped field electrodestructures 160 are arranged along lines. Stripe shaped gate structures150 are arranged between neighboring lines of needle-shaped fieldelectrode structures 160.

The IGFET 501 of FIG. 4D includes needle-shaped field electrodestructures 160 and a grid-like gate structure 150, wherein the fieldelectrode structures 160 are arranged in the meshes of the grid formedby the gate structures 150.

FIG. 4E is a schematic vertical cross-sectional view of thesemiconductor device of FIG. 4A through a transition region 691 in aplane parallel to the cross-sectional plane of FIG. 4A. The transitionregion 691 directly adjoins the transistor cell region 610 and includesfield electrode structures 160 as well as p wells 115 b in mesa sections170 between the field electrode structures. Gate structures 150 extendfrom the transistor cell region into the transition region 691.

The transition region 691 is devoid of any source zones or of suchsource zones that are electrically connected to the first loadelectrode. A metal control electrode 330 is formed on the interlayerdielectric 200. Gate contacts 315 g extend from the control electrode330 through the dielectric layer 220 and through the mask opening 214 inthe alignment layer 210 to or into the gate electrodes 155. The maskopenings 214 in combination with a selective etch of the dielectriclayer 220 relax the alignment conditions for a mask defining contacttrenches for the gate contacts 315 g in the interlayer dielectric 200.Auxiliary spacers 230 along sidewalls of the mask openings 214 maysupport a guided etch through a portion of the interlayer dielectric200.

FIG. 5 refers to a method of manufacturing a semiconductor device. Firsttrenches are formed that extend from a process surface into asemiconductor layer (702). An alignment layer with mask pits in a withrespect to the process surface vertical projection of the first trenchesis formed on the process surface, wherein sidewalls of the mask pitshave a smaller tilt angle with respect to the process surface thansidewalls of the first trenches (704). The mask pits are filled with anauxiliary material (706). In a mesa section of the semiconductor layerbetween the first trenches, a gate trench for a gate structure is formedby using the auxiliary material as an etch mask (708).

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a variety of alternate and/or equivalent implementations may besubstituted for the specific embodiments shown and described withoutdeparting from the scope of the present invention. This application isintended to cover any adaptations or variations of the specificembodiments discussed herein. Therefore, it is intended that thisinvention be limited only by the claims and the equivalents thereof.

The invention claimed is:
 1. A semiconductor device comprising: a gatestructure extending from a first surface of a semiconductor portion intoa mesa section between neighboring field electrode structures; and analignment layer formed on the first surface, wherein the alignment layercomprises mask pits formed in the alignment layer in a verticalprojection of the field electrode structures, wherein sidewalls of themask pits have a smaller tilt angle with respect to the first surfacethan sidewalls of the field electrode structures, wherein the gatestructure is in the vertical projection of a gap between neighboringmask pits.
 2. The semiconductor device of claim 1, wherein each of thefield electrode structures comprises a field electrode and a fielddielectric that separates the field electrode from a material of thesemiconductor portion.
 3. The semiconductor device of claim 2, furthercomprising: a buried contact formed at a distance to the first surfaceand directly adjoining the mesa section and one of the field electrodes.4. The semiconductor device of claim 1, wherein the tilt angle of thesidewalls of the mask pits with respect to the first surface is between30 degree and 60 degree.
 5. The semiconductor device of claim 1, whereinthe alignment layer is formed by high density plasma deposition ofsilicon oxide.
 6. The semiconductor device of claim 1, furthercomprising: an auxiliary spacer formed along sidewalls of openings inthe alignment layer between neighboring mask pits.
 7. A semiconductordevice, comprising: first trenches extending from a first surface into asemiconductor layer; an alignment layer on the first surface andcomprising mask pits formed in the alignment layer in a verticalprojection of the first trenches, wherein sidewalls of the mask pitshave a smaller tilt angle with respect to the first surface thansidewalls of the first trenches; and a gate trench formed in a mesasection of the semiconductor layer between the first trenches.
 8. Thesemiconductor device of claim 7, further comprising: a buried contact ata bottom of the first trenches.
 9. The semiconductor device of claim 8,wherein each buried contact comprises a conductive material recessedbelow the first surface.
 10. The semiconductor device of claim 7,wherein the first trenches are etched into the semiconductor layer. 11.The semiconductor device of claim 7, wherein the first trenches arevertically aligned with recessed subsurface structures formed in thesemiconductor layer.
 12. The semiconductor device of claim 11, whereinthe recessed subsurface structures are conductive structures recessedbelow the first surface.
 13. The semiconductor device of claim 11,wherein the recessed subsurface structures are insulator structuresrecessed below the first surface.
 14. The semiconductor device of claim11, wherein the recessed subsurface structures comprise field electrodesseparated from a material of the semiconductor layer by a fielddielectric.
 15. The semiconductor device of claim 7, wherein thealignment layer completely fills the first trenches.
 16. Thesemiconductor device of claim 7, further comprising: a gate electrode inthe gate trench; and a gate dielectric insulating the gate electrodefrom the mesa section of the semiconductor layer.
 17. The semiconductordevice of claim 7, further comprising: field electrode structures,wherein the first trenches are laterally interposed between the fieldelectrode structures and the gate trench.
 18. The semiconductor deviceof claim 17, wherein each of the field electrode structures comprises afield electrode and a buried contact laterally extending from the fieldelectrode to the mesa section of the semiconductor layer.
 19. Thesemiconductor device of claim 7, further comprising: an auxiliary spacerformed along sidewalls of openings in the alignment layer betweenneighboring mask pits.
 20. A semiconductor device, comprising: aplurality of transistor cells each of which comprises: a field electrodestructure or a portion of a field electrode structure extending from afirst surface into a semiconductor portion; at least one dopedsemiconductor region formed in a mesa section of the semiconductorportion; and a gate structure formed in the mesa section and comprisinga gate electrode insulated against the mesa section by a gatedielectric; an interlayer dielectric on the first surface and the fieldelectrode structures; and a first load electrode on the interlayerdielectric, wherein the interlayer dielectric comprises an alignmentlayer and a dielectric layer, wherein the alignment layer has mask pitsin a vertical projection of at least portions of the field electrodestructures, wherein sidewalls of the mask pits have a smaller tilt anglewith respect to the first surface than sidewalls of the field electrodestructures, wherein between neighboring mask pits, an opening in thealignment layer extends in a vertical direction through the alignmentlayer, wherein the openings are in the vertical projection of the gatestructures, wherein the dielectric layer fills the mask pits and theopenings.